1. Field of the Invention
The present invention relates to Residential Ethernet capable of simultaneously and efficiently providing real time service and non-real time service using Ethernet, and more particularly to a method for maintaining the starting point of a superframe in Residential Ethernet.
2. Description of the Related Art
Ethernet is the most widely used local area network technology and is now defined as a standard in an Institute Electrical of Electrical and Electronics Engineers (IEEE) 802.3. Ethernet has been originally developed by Xerox and has been advanced by technology companies such as Xerox, Digital Equipment Corporation (DEC), Intel, etc.
The Ethernet is a technology generally used when data is transmitted among a plurality of terminals or users. In conventional Ethernet competitive access is accomplished by means of a carrier sense multiple access/collision detect (CSMA/CD) protocol stipulated in the IEEE 802.3 standard. Typically a service frame of an upper layer is converted to an Ethernet frame while maintaining an inter frame gap (IFG), and the Ethernet frame is then transmitted. The Upper service frames are transmitted according to the creation sequence, regardless of the frame type.
Such conventional Ethernet has been known to be insufficient for transmitting a moving picture or voice data because of transmission delays. However, recently, various forms of research is being actively conducted to develop technology for transmitting synchronous data, such as image/voice data, by using the existing Ethernet. Such an Ethernet for transmitting synchronous data, which is currently under discussion, is referred to as “Residential Ethernet”.
In Residential Ethernet, frames are transmitted in a cycle unit, and generally, one cycle is defined as 125 μsec. One transmission cycle is divided into a synchronous section for transmitting synchronous frames and an asynchronous section for transmitting asynchronous frames. Herein, the synchronous frames refer to Ethernet frames having a fixed length, and the asynchronous frames refer to Ethernet frames having variable lengths.
Current Residential Ethernet restricts the maximum number of synchronous frames to sixteen in one superframe so that at least one asynchronous frame can be transmitted during the asynchronous section of the superframe. A maximum of 2153 bytes can be transmitted during the asynchronous section of the superframe.
FIG. 1 is a view illustrating the structure of a transmission cycle in Residential Ethernet.
The Residential Ethernet currently being discussed has a transmission cycle 10 of 125 μsec for data transmission, and each transmission cycle includes an asynchronous frame section 110 for transmission of asynchronous data and a synchronous frame section 100 for transmission of synchronous data.
More specifically, the synchronous frame section 100 for transmission of synchronous data has the highest priority in the transmission cycle, and includes 738-byte sub-synchronous frames 101, 102, and 103 according to a proposal under current discussion.
In addition, the asynchronous frame section 110 for transmission of the asynchronous data includes sub-asynchronous frames 111, 112, and 113 having various lengths in each corresponding area.
As shown in FIG. 1, it is necessary in Residential Ethernet to maintain an exact cycle because transmission is performed based on a cycle. However, it is difficult to maintain an exact cycle in Residential Ethernet because asynchronous frames have various lengths.
FIG. 2 is a view for explaining a case in which synchronization is not achieved due to asynchronous frames in Residential Ethernet.
Referring to FIG. 2, cycles 21, 22, and 23 include synchronous frames 201, 202, 203, 207, 208, 209, 212, and 213 and asynchronous frames 204, 205, 206, 210, and 211, all of which are transmitted.
The Residential Ethernet transmits synchronous data in synchronization with starting points of the cycles 21, 22, and 23. However, in FIG. 2, the synchronization of the cycles is disrupted due to the asynchronous frame 206 of the Nth cycle 21. Accordingly, the starting point of the (N+1)th cycle 22 is delayed by Δt1 214, and the starting point of the (N+2)th cycle 23 is delayed by Δt2 215. As described above, since asynchronous frames have various lengths, it is difficult to insert the asynchronous frames into every cycle to correspond exactly to the size of each cycle, it is difficult to achieve an exact synchronization of the frames.
Such a delay phenomenon in the start of a superframe occurs more frequently as the amount of asynchronous traffic becomes larger, and a delay time period becomes longer as the length of a transmitted asynchronous frame becomes longer.
As described above, the Residential Ethernet has a problem in that an asynchronous frame transmitted in an asynchronous section may cause delay in the starting point of the next superframe, and at the worst, a cycle may be delayed during a transmission time period for a maximum of 1518 bytes. Particularly, such delay may reduce the synchronous section of the next superframe.
In order to solve these problems, a hold scheme, a fragmentation scheme, and a RUNT scheme have been proposed. According to the hold scheme, when it is impossible to transmit an asynchronous frame within a transmission region of a cycle, the corresponding transmission region remains empty, and the data of the asynchronous frame is transmitted in the next cycle. According to the fragmentation scheme, when it is impossible to transmit an asynchronous frame within a transmission region of a cycle, the asynchronous frame is fragmented so as to include an asynchronous frame piece suitable to the corresponding transmission region, and the remaining pieces of the asynchronous frame are transmitted in the next cycle. According to the RUNT scheme, which is executed without consideration of transmission regions, if a new cycle starts while an asynchronous frame is being transmitted, the transmission of the corresponding asynchronous frame is stopped, and the corresponding asynchronous frame is again transmitted at the beginning of the asynchronous section in the next cycle.
The hold scheme among these schemes will now be described in more detail with reference to FIG. 3.
FIG. 3 is a view illustrating the structure of a transmission cycle based on the hold scheme for strict synchronization in Residential Ethernet.
Cycles 31, 32, and 33 include synchronous frames 301, 302, 303, 306, 307, 308, 310, 311, and 312 and asynchronous frames 204, 205, and 309, which are transmitted.
Referring to FIG. 3, it can be understood that synchronization for the starting point of each transmission cycle is achieved, differently from transmission cycles shown in FIG. 2. Such synchronization is achieved by controlling the transmission of asynchronous frames. In detail, in the case of the Nth cycle 31, there is an available transmission region in the Nth cycle 31 after the asynchronous frame 305 has been transmitted, but the available transmission region is smaller than the size of the next asynchronous frame 309. In this case, the transmission is controlled such that the available transmission region is left empty and the next asynchronous frame 309 is transmitted in the next cycle (N+1)th cycle 32, so that synchronization can be strictly maintained.
As described above, according to the hold scheme, when it is determined through comparison that the size of an asynchronous frame “A” is larger than the size of an available asynchronous-frame transmission region “B” in a transmission cycle, the transmission cycle is transmitted with the available transmission region “B” left empty, and the asynchronous frame “A” is transmitted in the next cycle.
However, the above hold scheme is illustrated only with respect to a case in which one Residential Ethernet node transmits asynchronous frames sent from one asynchronous device. Therefore, if it is assumed that an Residential Ethernet node transmits asynchronous frames sent from a plurality of asynchronous devices, the construction and operation of the Residential Ethernet node will differ from those described with reference to FIG. 3.
FIG. 4 is a block diagram illustrating the construction of a Residential Ethernet node to which the hold scheme for strict synchronization is applied.
The Residential Ethernet node 41, to which the hold scheme for strict synchronization is applied, includes a synchronous queue 401, an asynchronous queue 402, and a multiplexer 403. The synchronous queue 401 receives and temporarily stores synchronous data, so as to transmit the synchronous data by inserting the synchronous data into a cycle. The asynchronous queue 402 receives asynchronous frames from different legacy LAN devices 42 and 43 and temporarily stores the asynchronous frames, so as to transmit the asynchronous frames by inserting the asynchronous frames into a cycle. The multiplexer 403 receives synchronous frames and asynchronous frames from the synchronous queue 401 and asynchronous queue 402, and transmits the synchronous frames and asynchronous frames in a form of a transmission cycle.
Herein, the asynchronous frames are received from the legacy LAN devices 42 and 43 and are stored in the asynchronous queue 402. That is, asynchronous frames 411 and 412 transmitted from the first legacy LAN device 42 and asynchronous frames 421 and 422 transmitted from the second legacy LAN device 43 are stored in the asynchronous queue 402.
When the hold scheme is employed, the first-stored 1-1 asynchronous frame 411 must be primarily transmitted and then secondly-stored 2-1 asynchronous frame 421 must be transmitted during a first/next transmission cycle. However, when the size of an available transmission region remaining after the first-stored 1-1 asynchronous frame 411 is smaller than that of the secondly-stored 2-1 asynchronous frame 421, the remaining transmission region is left empty, and the secondly-stored 2-1 asynchronous frame 421 is transmitted in the next transmission cycle.
Such a transmission method is efficient when the same type of asynchronous frames (i.e. asynchronous frames having the same destination address and the same source address) are transmitted, because it is necessary to sequentially transmit all the asynchronous frames. However, there exists a need to develop a new transmission method which can actively reduce such a waste of bandwidth when different types of asynchronous frames having different destination addresses or different source addresses are transmitted.